1. Field of the Invention
This invention relates to an improved method for the preparation of fibers or films with excellent mechanical strengths from phenolic resins.
2. Description of the Prior Art
There is well known a method for the preparation of fibers or films from thermoplastic novolac resins which are obtained by interacting phenols and formaldehyde under acidic conditions, in which method the novolac resins are melt spun into fibers or formed into films, and then cured by heating the fibers or films in a formaldehyde environment to obtain infusible, cured novolac fibers or films. Further, it is known that a resol which is obtained by reacting a phenol with formaldehyde in the presence of basic catalyst can be formed into a fiber by subjecting it to spinning and subsequently to a mere heat curing treatment. The fibers or films obtained from these phenolic resins have a high carbon content and a three dimensional cross-linked structure, thus being infusible and incombustible, and very excellent in chemical resistance. The phenolic fibers have been used extensively in a wide variety of fields such as of antiflaming wear, working wear, inwall clothes, tents, various kinds of filters, separators for storage battery, felts, curtains and reinforcing materials for lamination.
However, the phenolic fibers or films obtained by the conventional methods are somewhat insufficient in mechanical strength. In order to improve the mechanical strength of the phenolic resin fibers, there has been proposed a method wherein the fibers are stretched in an aqueous methanol solution and thermally set. This method is disadvantageous from a practical viewpoint in that elongation of the resultant fibers is reduced and the fibers become brittle though the mechanical strength is increased. There is accordingly a strong demand of a method for preparing phenolic fibers or films which are improved in mechanical strength and elongation without loss of other useful properties.